I have hoisted my boat up in the garage to inspect the c/b in its deployed position.
On inspection it would appear to have a slight swist to the trailing edge. The twist amounts to a lateral deviation of about 1/4in over the length. The plate is otherwise sound, is vertically straight and does not jam in the slot.

Not ideal I'm sure but is this unusual and is it a case that I will need to replace in order to remain competitive?

Yes and yes.

What boat?

Does it matter a twisted centreboard is a twisted centreboard on any boat what ever type let alone Mk of Merlin rocket.

Logically if you were competitive before you knew about it you should, unless affects you mentally, remain equally competitive now you know about it. In practice I'm convinced boat speed has a huge psychological component, and if you think you're slow you will be. 
As for what difference it actually makes to boat speed: its very difficult to work it out unless you do really intensive two boat tuning with a partner. The performance will probably be slightly different on one tack to the other, but its just possible performance on one tack might be slightly improved. The overall effect is likely to be down. 

As to how much - well, lets try some sums. it will doubtless depend on where you sail, but race performance is probably 90% the sailor 10% the boat. Of that 10% you might guess one third each rig hull and foils. Speed from foils could be 50/50 rudder and centreboard. If your board is smooth, fair and polished but a little bit assymettric one might guess that its 10% off a brand new foil. Do the sums and that's what, 0.16% drop in performance or 6 seconds over a one hour handicap race? If you're class racing its a bit more complicated than that because I think that, given say 50 boats starting level up the first beat then tiny differences in boatspeed are magnified because the faster boats will get really clear air sooner and the slower boats dirty air sooner...

My mkIX has a twisted board and it is very rapid. I was wondering whether it is twisted the right way to favour the reach at Hampton, in a south-westerly we spend a lot of time on port tack. I will have to check during the winter refurb. Uncle Mikey told us that Charge Of The Light Brigade had been built with a twisted centreboard case, he reckoned it was quicker on one tack, but it's record speaks for itself.

Does it matter what boat? Yes. A replacement for a IXb is going to be about £40 and you'll find a used one easily. For a Mk4 Winder you're looking at £400 and you need a brand new one.

Its a Chipstow tales. And I'll get a new board cos I need very little excuse to buy new stuff (except for hulls clearly).
Surely, although you would get asymetric lift (i.e towards one side more than the other) which may be deemed to be beneficial on one tack, you will also get a constant increase in drag on all points of sail due to the increase in longitudinal projected area of the twisted portion - no?

My 1962 BEL-1 OK Dinghy is asymmetric not only with the C'board, but with the hull. What it means is that one tack is better than the other, even regardless of wind shear, Coriolis force & all the rest. I just figure it will all balance out, but aim to make my (minor) conquests on the favoured tack.